Various sub-populations of mouse B and T cells have been identified that differ in their permissiveness to murine leukemia viruses (MuLV). We propose to study the expression of receptor for MuLV on the surface of permissive and non-permissive lymphocytes. Furthermore, we have noticed that a given MuLV strain may induce histopathologically different types of leukemias/lymphomas depending on the availability of specific target cells in the host. Various cellular humoral factors regulating the divergence of neoplasia will be studied. We will also study the cellular cooperations that apparently play a role in the generation of immunosuppressive cells in MuLV-infected mice.